1. Field of the Invention
The present invention relates to a rolling mill and rolling method for rolling a plate. Particularly, the invention is concerned with a rolling mill and rolling method using work rolls of a small diameter and suitable for rolling a hard or ultra-thin strip.
2. Description of the Prior Art
Heretofore, working rolls of a small diameter have been used for rolling a hard or ultra-thin strip such as stainless steel strip. With a decrease in diameter of the work rolls, the flexural rigidity of the rolls becomes lower inevitably. Particularly, deflection in a horizontal plane poses a problem. This horizontal deflection causes a more marked disturbance in the shape (flatness) of the strip used. The horizontal deflection sometimes exceeds the correction capacity of a shape correcting device such as the work roll bender which has heretofore been used. If the rolls deflect vertically in opposite directions, the central portions of the upper and lower work rolls undergo forces acting in opposite directions, which forces promote the vertical opposite deflections in an accelerative manner. In this case, if the rolling load is set large, the rolls may be broken. In order to prevent the occurrence of such a trouble, the application of a large load must be avoided.
In view of the above points, there have been developed Cluster mill type rolling mills, including Sendzimir mill, as well as a rolling mill having a horizontal deflection preventing mechanism wherein the drum portions of work rolls are supported horizontally with support rolls such as that disclosed in Japanese Patent Laid Open No.18206/85. In these rolling mills, however, since the support rolls are divided in the direction of the roll drum length, the surface properties of the plate rolled are deteriorated due to mark transfer by the divided support rolls.
As a rolling mill which takes into account the prevention of such deterioration in the plate surface properties and which permits the use of work rolls of a small diameter, there has been developed such a rolling mill as disclosed in Japanese Patent Laid Open No.50109/93. In this rolling mill, horizontal support rolls are mounted outside the area through which the maximum width of the strip to be rolled passes, then a horizontal deflection of each work roll is detected and a horizontal bending force of the roll is controlled so that the horizontal deflection thereof becomes equal to zero. At the same time, the work rolls are each moved to a position where the horizontal force which causes the horizontal deflection is zero. Since a horizontal support device for the strip is not present in the area where the strip passes, it is possible to prevent the deterioration of surface properties attributable to such horizontal support device.
In the rolling mill disclosed in the above unexamined publication 50109/93, however, a limit has so far been encountered in reducing the diameter of each work roll. More particularly, if the work roll diameter is set below a certain value, the flexural rigidity becomes extremely low, and the responsivity in horizontal deflection control also encounters a limit, resulting in that it becomes no longer possible to make the horizontal deflection zero. Actually, in the rolling mill of the type described in the above unexamined publication, it is considered that a roll diameter of 10% or so of the maximum strip width is the limit in reducing the roll diameter. It has been difficult to make the roll diameter still smaller.
For the simplification of a rolling mill and for the reduction in diameter of working rolls, a rolling mill having a support mechanism which prevents the deflection of work rolls on an incoming side of the rolls is disclosed in Japanese Patent Laid Open No.94509/84. The support mechanism is provided with a cooling means using adjustment of liquid pressure to prevent friction caused by the support. Each work roll is provided with a shifting mechanism so as to adapt itself to changes in rolling conditions (for example the plate to be rolled). According to such a technique, the deflection of work rolls can be prevented to some extent, but no consideration is given to diminishing the horizontal force. Besides, since the shifting mechanism is provided on the work roll itself, the deflection of the work roll is influenced by a shifting motion. It has so far been difficult to effect shifting while minimizing the deflection of each work roll.
Further, as a rolling mill suitable for preventing the foregoing deterioration of the strip surface properties and for attaining a further reduction in diameter of work rolls, a rolling mill provided with a horizontal support mechanism for work rolls, using static pressure bearings, is disclosed in Japanese Patent Publication No.13366/96. This rolling mill is constructed schematically as in FIG. 17 for example. As shown in the same figure, work rolls 102 and 103 for rolling a strip 101 are supported vertically by means of intermediate rolls 104,105 and back up rolls 106,107 and are supported horizontally by means of static pressure bearings 112,113,114 and 115 through idler rolls 108,109,110 and 111.
In the rolling mill disclosed in the Japanese patent publication 13366/96 such as that shown as an example in FIG. 17 in which the work rolls 102 and 103 are supported horizontally by means of static pressure bearings 112,113,114 and 115, there have been the following points to be further improved.
In the rolling mill shown in FIG. 17, the work rolls 102 and 103 are mounted centrally of the rolling mill. On the other hand, where the work roll diameter is reduced, it is impossible to drive the work rolls directly because a high strength of their driving shafts is to be ensured. It is unavoidable for the working rolls to be driven indirectly through back up rolls or intermediate rolls. Consequently, a tangential force acting in the horizontal direction is developed at the time of imparting a rolling torque to the work rolls through the back up rolls or the intermediate rolls. Under rolling conditions involving a large torque, the driving tangential force (horizontal force) also becomes large, so that an excessive force is also imposed on the static pressure bearings which bear the force. Therefore, if the fluid pressure fed to the static pressure bearings is not sufficient, it is impossible for the bearings to bear the horizontal force, with the result that the rolls and bearing pads of the static pressure bearings come into contact with each other and both are flawed. The flaws on the work rolls are inevitably transferred onto the strip being rolled, thus deteriorating the strip quality markedly. On the other hand, if the flaws on the bearing pads are left as they are, the fresh rolls after roll replacement will also be flawed. For this reason, it is necessary to replace the bearing pads themselves. This roll replacing work requires much time, thus leading to deterioration of the productivity. Besides, repair of the bearing pads costs much because the bearing pads require a high fabrication accuracy. Thus, the contact between the work rolls and the bearing pads of the static pressure bearings caused by the aforesaid excessive horizontal force results in great damage.
For preventing such an inconvenience as mentioned above, it is necessary to take an appropriate measure, for example, let the oil pressure fed to the static pressure bearings have a margin. To this end, it is inevitably required to use a pump and a tank, resulting in an increase of the equipment cost and of the power cost.